Liquid crystal displays can be classified to many numerous types according to the methods of manufacturing and the structures. For example, top gate type and bottom gate type can be deferentiated. For the manufacturing of top gate type LCD, active layer made with semiconductor material is formed before gate insulating layer and gate pattern are formed. But, in case of bottom gate type LCD, the gate pattern and the gate insulating layer is formed prior to the active layer.
On the other hand, the classification of reflective type and transmissive type can be possible. In the transmissive type LCD, the pixel electrode is made with reflective materials like metals which reflect light from external environment. But, in the reflective type LCD, the pixel electrode is made with transparent conductive materials containing indium metal oxide lineage like ITO (indium tin oxide). In the transmissive type LCD, independent light source is placed at the backside of LCD panel and thus called backlight, and various colorful images can be presented by controlling the state of liquid crystal of numerous pixels to transmit or to shied the light from the backlight.
Recently, reflective type LCD is considered as an alternative in notebook LCD which need large display size and high quality image. And reflective-transmission type LCD which has merits of both reflective type and transmission type is also presented by SHARP(registered trade mark).
In the reflective transmission type LCD of SHARP, transmissive pixel electrode is formed to be connected to source electrode of thin film transistor in the process of making thin film transistor on glass substrate. And passivasion layer which has contact hole in the source region of the thin film transistor is formed on the thin film transistor. Then, reflective metal layer like aluminum is formed on the passivation layer and patterned to confine the reflective area and to form a window which reveals some part of transmissive layer. Here, the patterned reflective metal, the reflective pixel electrode is connected to the source electrode through the contact hole.
Meanwhile, in the process of fabricating the reflective transmission type LCD, some problems can be caused according to the materials adopted as transmissive pixel electrode and reflective pixel electrode. For example, if the reflective pixel electrode made with aluminum, is electrically connected with the source electrode by the transmissive pixel electrode which covers the source electrode. But, the transmissive pixel electrode is made with ITO (indium tin oxide) generally used as transparent conduction layer, an oxide insulation layer is to be formed between the reflective pixel electrode and transmissive pixel electrode. Then, the oxide layer prevents the enhancement of electric field to the reflective pixel electrode.
To avoid the problem, a parameter conduction layer can be used at the interface of the two problematic electrode materials. Also, the two problematic pixel electrode may be connected to the source electrode at different positions to prevent the problem. However, compared with the relatively large pixel electrode area, the region of the source electrode is confined to relatively small area, which makes it difficult to design and to form the transmissive area and the reflective area properly and simply.